The mitochondrial genome of a 9-arm feather star Thaumatocrinus naresi (Crinoidea, Pentametrocrinidae)

Abstract The genus Thaumatocrinus is composed of species displaying 10 undivided arms arising from 10 radials. Thaumatocrinus naresi is also considered to be a 10-arm species, but individuals with 9 arms were also reported. Here, we report the mitochondrial genome of T. naresi with 9 arms collected from the western Pacific. The genome is 16,047 bp in length with a 67.84% AT content. It contains 13 protein-coding genes (PCGs), 2 ribosomal RNA genes, and 22 transfer RNA genes. Phylogenetic analysis shows that T. naresi forms an independent lineage within Comatulida.


Introduction
Thaumatocrinus Carpenter, 1883 is a genus feather star in the family Pentametrocrinidae A. H. Clark 1908, composed of 6 species with 10 arms (Carpenter 1883(Carpenter , 1884(Carpenter , 1888Clark 1907Clark , 1908Clark , 1915Clark and Clark 1967). Individuals with fewer arms in the genus Thaumatocrinus were also frequently spotted (Clark 1908(Clark , 1915. In addition, juvenile specimens of Thaumatocrinus were reported to have only 5 arms and radials (Carpenter 1884;Clark 1915). In this study, the complete mitochondrial genome from a 9-arm T. naresi is described, and a phylogenetic tree of T. naresi and other crinoid species is included.

Materials and methods
The specimen was collected alive from the western Pacific (162 49 0 43 00 E, 15 10 0 46 00 N, 1318 m depth) and identified as Thaumatocrinus naresi based on its morphological characters ( Figure 1).
The specimen (RSIOCR118) and its DNA are deposited in the Sample Repository of the Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China (Chunsheng Wang, wangsio@sio.org.cn). DNA was extracted with QIAamp Tissue Kit (QIAGEN, Hilden, Germany) and mitochondrial DNA was amplified with a DNA REPLI-g Mitochondrial DNA Kit (QIAGEN, Hilden, Germany). Library construction and sequencing were performed by Biozeron (Biozeron, Shanghai, China) using the Illumina novaseq 6000 sequencing platform (Illumina, San Diego, CA, USA). The raw data was assembled using GetOrganelle v1.7.5 (Freudenthal et al. 2020), and genome annotations were performed using MITOS (Bernt et al. 2013). The phylogenetic analysis was conducted by the maximum likelihood (ML) method using MEGA 11 (Tamura-Nei model) with 1000 replication of bootstrap assembling (Tamura et al. 2021).

Discussion and conclusion
The phylogenetic relationship of T. naresi with other crinoids was analyzed. 12 mitochondrial genomes were obtained from the GenBank database (9 from Crinoidea, 2 from Ophiuroidea, and 1 from Asteroidea). The tree topologies showed that T. naresi fell within the cluster comprising Comatulida species and formed an independent lineage ( Figure 2) with a high bootstrap value (100). In this research, the complete mitochondrial genome sequence of T. naresi was provided. Given that abundant mitochondrial genomic data and comprehensive analysis are still required to discover  the phylogeny and evolution of crinoids, this study is an important contribution to this field ( Figure 3).

Ethical approval
Study species were crinoids collected in International Waters and permission for sampling was not necessary. Animals were preserved in ethanol.

Author contributions
Qinyi Lin was involved in the conception, design, analysis of the data and drafting the paper; Ruiyan Zhang and Chunsheng Wang were involved in the conception, revising it and the final approval of the version to be published; and all authors agree to be accountable for all aspects of the work.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Data availability statement
The genome sequence data that support the findings of this study are openly available in GenBank of NCBI at [https://www.ncbi.nlm.nih.gov] under the accession no. OP428702. BioProject, SRA, and Bio-Sample numbers are PRJNA880019, SRR21615206, and SAMN30825162, respectively.